The present invention relates to an optical access network system that transmits high-speed optical signals to subscribers, and more particularly to an optical access network system that receives a multiplexed optical signal of a plurality of optical signals having different wavelengths from a broadband optical network, and transmits optical signals having specified wavelengths to subscribers.
Various methods have been proposed as a next-generation optical access network system that transmits optical signals to a subscriber as is without performing electrical conversion. The work of global standardization is advancing, and as examples of such an access system, there is (1) E-PON (Ethernet PON Ethernet frames that is obtained by modifying a B-PON (Broadband Passive Optical Network) for ATM frames, (2) GE-PON (Giga Ethernet PON) for Giga Bit Ethernet, and (3) Unique-standard G-PON. These PON systems do not use active elements that perform electrical-to-optical conversion in the network, such as in the case of ADS (Active Double Star), but rather comprise only passive optical parts, which make it possible for a low-cost End-to-End optical network. Moreover, even though equipment cost is high, adoption of a WDM-PON that uses a Wavelength Division Multiplex (WDM) signal for larger capacity is being investigated because it lowers the unit cost per bit. Ethernet is a registered trademark.
FIG. 16 is a drawing of a conventional GE-PON, in which a center station 2 that is connected to the Internet or Broadband Ethernet 1 is connected to users 3 by optical paths 5, 61, 62, 63, . . . via an optical coupler 4. An Optical Line Terminal (OLT), which is the center station 2, has a 100/1000 BASE-T or 1000 BASE-SX/LK interface. A 10/100 BASE interface subscriber unit (Optical Network Unit: ONU) 3a and a personal computer 3b, which is the user terminal that is connected to the ONU, are located at subscribers' homes 31, 32, 33, . . . .
The maximum distance between the center station 2 of the communication carrier and the users 31, 32, 33 (distance between access intervals) is approximately 20 km, and by using a wavelength of 1490 nm for DOWN optical signals, and a wavelength of 1310 nm for UP optical signals, 1 Gbps communication is performed in both directions simultaneously.
The center station 2 transmits Ethernet frames that were received from the Internet or broadband Ethernet 1 and destined for users 31, 32, 33 to an optical transmission path 5. The optical coupler 4 splits the DOWN Ethernet frames to a maximum of 32 branches, and transmits them to optical transmission paths 61, 62, 63, . . . , and the ONU unit 3a of each user 31, 32, 33, . . . extracts out its own respective packet, and inputs it to the personal computer 3b. Furthermore, the ONU 3a converts the transmission data from the personal computer 3b of each user 31, 32, 33, . . . to an optical signal and transmits the signals to the optical transmission paths 61, 62, 63, . . . , and the optical coupler 4 combines the signals and sends them to the center station 2.
FIG. 17 is a drawing of a conventional WDM-PON, where the maximum distance between the communication carrier and user is approximately 20 km, and communication is performed by multiplexing 32 optical signals having different wavelengths. The wavelengths of the optical signals that are used for UP and DOWN communication are the same, λ1 to λ32, and DOWN and UP communication are performed alternately (ping-pong transmission). A transmission unit (Tx) at a relay station of the communication carrier multiplexes 32 optical signals λ1 to λ32 having different wavelengths that were modulated with the data destined for each subscriber at an optical multiplexer (MUX) 3a, and outputs the multiplexed signal to a transmission path 7 via an optical circulator 3b. An optical multiplexing/demultiplexing filter (MUX/DMUX) 8 receives a Wavelength-Division Multiplexed optical signal (WDM optical signal), and divides that signal into optical signals λ1 to λ32 for each wavelength, and inputs the signals to the ONU units 91 to 932, which are the subscriber units. The ONU units 91 to 932 convert the signals to electrical signals and input the signals to the personal computers (not shown in the figure). Moreover, the personal computers of the subscribers input transmission data to the ONU units 91 to 932, then those ONU units 91 to 932 modulate and transmit the signals of the wavelengths λ1 to λ32, and the optical multiplexing/demultiplexing filter 8 combines the signals having different wavelengths that were output from the ONU units 91 to 932, and outputs the result to the transmission path 7. The optical circulator 3b located at the relay station of the communication carrier inputs the received optical signal to an optical demultiplexing filter (DMUX) 3c of a receiving unit (Rx), and that demultiplexing filter 3c separates and outputs optical signals λ1 to λ32 for each wavelength.
Moreover, as related art, there is a communication network in which PONs are connected in a ring shape such that transmission is possible in both directions, with a plurality of terminals connected to each PON such that communication can be performed among the terminals (Refer to WIPO No. WO95/19689 (Japanese Patent Publication No. H9-510053)). In the prior optical communication network, communication channels between terminals are multiplexed using various wavelengths, and use wavelengths that are assigned by a wavelength assignment unit to perform communication between terminals.
In the case of a PON system, a network can be created using inexpensive passive optical elements, however, there is a problem in that it is possible to use only a bandwidth of approximately 30 to 100 Mbps per subscriber, and high-speed communication exceeding 100 Mbps is not possible. Moreover, in a PON system, the network is constructed such that branching is performed by an optical coupler, so the same signal is distributed to all of the subscriber units that are connected to that optical coupler. Therefore, the danger of communication contents being decoded increases, making it impossible to maintain physical secrecy, and thus in a certain sense, the system can be said to be the same as a public wireless LAN. As can be seen from the above explanation, a conventional PON system has the disadvantage in that there is insufficient security for financial or business uses.
On the other hand, in a WDM-PON, it is possible to assign one wavelength for each user, and thus it is possible to maintain physical secrecy on the ONU side. However, since WDM optical signals are used, the number of users that can use a system is limited to a few tens of users due to restrictions of the number of multiplexed wavelengths, and to build a WDM-PON system, an enormous amount of fiber optic cables which connect between the communication carrier and users is necessary, which is reflected in the cost of the fiber optic equipment. On the ONU side as well, when WDM optical signals are used, signal wavelength management of each individual ONU is necessary, which increases the cost of the ONU terminal. In addition, in a WDM-PON, it is possible to maintain sufficient bandwidth, however, wasted bandwidth occurs due to ping-pong transmission.